Added a --user option to README

[siefkenj]

For those without root permissions, the --user option can be used with the python setup.py install command to install a local-only copy.

[JeffFessler]

On my Mac (OS 10.14.6 Mojave) with python 3.8.5 this very helpful tip led to installation of text2qti in
~/Library/Python/3.8/bin rather than in ~/.local/.

[molpopgen]

In a lot of cases, using pip to do a local install is preferable, as it automatically does it locally for users:

pip3 install .

or

python3 -m pip install .

The latter form lets you be more explicit about which Python you are using, in case you have multiple installs of python 3.

[gpoore]

Thanks for all the suggestions about alternate ways to install the development version! I've expanded the README to cover setup.py with --user as well as pip.